Apparatus for automatically feeding sheets

ABSTRACT

A stack of sheets such as documents to be fed one by one from the lowermost sheet is clamped between a lower feeding roller rotated in a given direction and an upper pressing roller whose shaft is loosely inserted in an elongated hole formed in a bearing secured to a rotatable arm. A coiled spring is connected between a free end of the arm and a fixed body of the apparatus to rotate the arm in such a manner that the pressing roller is urged against the stack of sheets with a large force due to the strong restoring force of the spring and the gravitational force of the pressing roller. A stopper plate is arranged in a rotational movement path of the arm at such a position that when a thickness of the stacked sheets has decreased to a given value, the rotation of the arm is inhibited by an engagement of the arm and stopper plate. After that engagement, only the pressing roller moves downward along the elongated hole and the pressing roller is urged against the stack of sheets with a small force due only to the gravitational force of the pressing roller.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus for automatically feedingsheets one by one from a stack of sheets.

Such a sheet feeding apparatus can be used in a facsimile unit, anelectrophotographic copying machine and the like. For instance, in anelectrophotographic copying machine the above sheet feeding apparatus issometimes provided as an automatic document feeder which can feed thestacked documents one by one onto a template for projecting an image ofa document onto an electrophotographic photosensitive member such as aphotosensitive drum.

FIG. 1 shows an embodiment of a known document feeding apparatuscomprising a document feed mechanism 5 having a feeding roller 1 rotatedin the direction shown by an arrow, a pressing roller 2 arrangedopposite to the feeding roller 1 and an arm 4 pivotally supported by ashaft 3. The pressing roller 2 is rotatably supported on the free end ofthe arm 4. At an upstream position, viewed in the document feeddirection, is provided a document separating mechanism 6. A stack ofdocuments 8 to be fed one by one is placed on a document table 7 and isclamped between the feeding roller 1 and the pressing roller 2. When thefeeding roller 1 is rotated, the stack of documents 8 is fed toward thedocument separating mechanism 6 which comprises a separating roller 9rotated in the direction shown by an arrow, a pressing plate 10 arrangedin an inclined fashion with the sheet feed direction and a frictionalsheet 11 applied on that surface of the plate 10 which is facing theroller 9. Then the lowermost document is separated from the stack ofdocuments 8 and is further fed forward by means of the feeding rollers 1and 9, while the remaining documents are retained in position by theseparating mechanism 6. In this manner the documents are fedsuccessively from the lowermost one in the stack of documents 8. Duringthis feeding operation, the arm 4 is rotated about the shaft 3 in thecounter-clockwise direction in accordance with the decrease in thethickness T of the document stack 8 so that the variation of thethickness T, i.e., the number of documents in the stack 8, can becompensated for.

In such a known apparatus the stack of documents 8 is clamped betweenthe feeding roller 1 and the pressing roller 2 only with a small forcedue to the gravitational force of the roller 2 and arm 4, and thusbetween the lowermost document and the feeding roller 1 there is notproduced a sufficiently large frictional force for feeding the lowermostdocument. Particularly when the number of stacked documents 8 is large,there might be produced between the lowermost document and the documenttable 7, a somewhat large frictional force which is sometimes largerthan that produced between the feeding roller 1 and the lowermostdocument, and thus, the document could not be fed positively and stably.

In order to avoid such a disadvantage it has been proposed, as shown inFIG. 2, to arrange a coiled spring 12 between the arm 4 and a fixedmember so as to increase the frictional force generated between thelowermost document and the feeding roller 1. However, in such a knownapparatus, when the number of the stacked documents 8 becomes small,there might be produced a serious problem, which will be explainedhereinbelow. Such a problem becomes noticeable when the document is athin sheet such as tracing paper.

When the number of the stacked documents 8 is large as illustrated inFIG. 3A, the sheets are smoothly fed one by one by the feeding andseparating mechanisms 5 and 6 even if the documents are thin. However,since the stacked documents are compressed between the feeding roller 1and the pressing roller 2 by means of the relatively strong restoringforce of the spring 12, a large frictional force is produced between thesuccessive documents. Therefore, when the lowermost document 8a is fedby the feeding rollers 1 and 9 as shown in FIG. 3B, the remainingdocuments above the lowermost document 8a are also advanced togetherwith the lowermost sheet 8a. However, the advance of these documents issuppressed by the friction plate 11 and thus they start bending upwardas illustrated in FIG. 3B. The degree of this bending becomes larger asshown in FIGS. 3C and 3D in accordance with the advance of the lowermostdocument 8a. When the documents are bent to a great extent as shown inFIG. 3D, so-called jamming is produced and the documents are clogged inthe feeding apparatus. When such jamming is produced, the documents,i.e., valuable originals, are damaged to an impermissible extent.Further, once the jamming occurs, the operation of the apparatus shouldbe stopped and the jammed documents must be removed carefully. Thisresults in troublesome work and lack of reliability.

SUMMARY OF THE INVENTION

The present invention has for its object to provide a novel and usefulautomatic sheet feeding apparatus which can effectively avoid the abovementioned drawbacks of the known apparatuses and can feed stacked sheetsone by one in a positive and stable manner by clamping the stackedsheets with a larger force when the stack of sheets has a largerthickness, but with a smaller force when the thickness of the stackbecomes smaller.

According to the invention an automatic sheet feeding apparatuscomprises

means arranged below a stack of sheets for feeding the sheets one by onefrom the lowermost sheet which is made to contact the feeding means;

means arranged above the stack of sheets for pressing the stack ofsheets against the feeding means with a first force when the thicknessof the stack of sheets is larger than a predetermined value, and with asecond force which is smaller than said first force when the thicknessof the stack of sheets is at most equal to said predetermined value; and

means arranged at a downstream position viewed in a sheet feed directionfor separating the lowermost sheet from the stack of sheets to feed thethus separated sheet forward.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section showing an embodiment of a known automaticsheet feeding apparatus;

FIG. 2 is a cross section illustrating another embodiment of a knownautomatic sheet feeding apparatus;

FIGS. 3A to 3D are cross sections for explaining how a jam is producedin the known apparatus shown in FIG. 2;

FIGS. 4A and 4B are cross sections showing an embodiment of theautomatic sheet feeding apparatus according to the invention;

FIGS. 5A to 5C are cross sections and schematic views showing anotherembodiment of the automatic sheet feeding apparatus according to theinvention;

FIG. 6 is a cross section depicting another embodiment of the automaticsheet feeding apparatus according to the invention; and

FIG. 7 is a cross section showing still another embodiment of theautomatic sheet feeding apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 4A and 4B show an embodiment of the automatic sheet feedingapparatus according to the invention. The apparatus comprises a feedingroller 21 which is arranged below a stack of sheets 28 placed on a sheettable 27 and is rotated in the direction shown by an arrow. The stack ofsheets 28 is urged against the feeding roller 21 by means of a pressingroller 22 rotatably supported by an arm 24 which is journaled by a shaft23. The feeding roller 21, the pressing roller 22 and the arm 24constitute a sheet feeding mechanism 25 together with a coiled spring 32provided between a free end of arm 24 and a fixed member. At adownstream position with respect to the sheet feeding mechanism 25 isarranged a sheet separating mechanism 26 comprising a separating andfeeding roller 29 rotated in the direction shown by an arrow, a pressingplate 30 and a friction sheet 31 applied on the pressing plate 30.

The construction so far explained is same as that of the known apparatusshown in FIG. 2. According to the present invention, the shaft 33 of thepressing roller 22 is inserted into an elongated hole 34a formed in abearing 34 secured to the arm 24 and thus, the shaft 33 can move alongthe elongated hole 34a. At the same time there is arranged a stopper 35in a rotational locus of the arm 24 at such a position that when thethickness T of the stacked sheets 28 becomes a predetermined valueT_(R), the arm 24 is in contact with the stopper 35. As illustrated inFIG. 4A when the thickness T of the stacked sheets 28 is larger than thepredetermined value T_(R), i.e., the number of the stacked sheets islarger than the predetermined number, the arm 24 is lifted by the rollershaft 33 and is not brought into contact with the stopper 35, and thusthe shaft 33 of the pressing roller 22 is engaged with the upper edge ofthe elongated hole 34a of the bearing 34. In this manner, the roller 22is urged against the stacked sheets 28 with a strong force due to therestoring force of the spring 32 and the gravitational force of theroller 22 and arm 24. Therefore, a sufficiently large frictional forceis produced between the feeding roller 21 and the lowermost sheet forfeeding the stack of sheets 28 forwardly, even if the number of sheetsis large and the stack of sheets is heavy. The stacked sheets 28 are fedinto the sheet separating mechanism 26 and are further fed one by onefrom the lowermost sheet.

As the sheets are successively fed and the thickness T of the stack ofsheets becomes the given value T_(R), the arm 24 comes into contact withthe stopper 35. After that the arm 24 can not rotate in thecounter-clockwise direction and then only the pressing roller 22 movesdownward, because its shaft 33 is supported in the elongated hole 34a ofbearing 34. Therefore, after the thickness of the stacked sheets 28 hasdecreased to the predetermined value T_(R), the restoring force of thecoiled spring 32 and the gravitational force of the arm 24 are no longerexerted upon the pressing roller 22 and thus, the pressing roller 22 isurged against the stack of sheets 28 with a small force due only to itsgravitational force. In this manner, according to the invention, thesheets can be effectively prevented from being bent and jamming can befully avoided.

FIGS. 5A to 5C show another embodiment of a sheet feeding apparatusaccording to the invention. In this embodiment portions similar to thoseof the previous embodiment are denoted by the same reference numeralsused in FIGS. 4A and 4B. In this embodiment, use is made of a pressingplate 40 instead of the pressing roller. That is, the pressing plate 40is connected to a pair of arms 24a and 24b which are pivotally supportedby a pair of shafts 23a and 23b, respectively. The front edge 40a of thepressing plate 40 is bent upward so as to ease the insertion of a stackof sheets 28 between the feeding roller 21 and the pressing plate 40.The shafts 23a and 23b are secured to side walls 41a and 41b of theapparatus, respectively. Along the side walls 41a and 41b are arrangedlevers 42a and 42b, respectively, in such a manner that the levers arerotatable about pins 43a and 43b, respectively. Free ends of the levers42a and 42b are bent so as to form projections 44a and 44b,respectively, and these projections extend inwardly through openings 45aand 45b which are formed in the side walls 41a and 41b, respectively, tosuch an extent that the projections 44a and 44b extend into therotational loci of the arms 24a and 24b and can be made to contact theupper edges of the arms. The openings 45a and 45b have such dimensionsthat the arms 24a, 24b can rotate in the clockwise direction in FIG. 5Ato a sufficiently large angle for allowing the insertion of the stack ofsheets 28 between the feeding roller 21 and the pressing plate 40. Asbest shown in FIG. 5C, between the levers 42a and 42b and a fixed memberare arranged coiled springs 46a and 46b, respectively, and therefore thelevers 42a, 42b and thus the arms 24a, 24b, are biased to rotate in thecounter-clockwise direction.

When the thickness of the stacked sheets 28 is thicker than thepredetermined value, the arms 24a, 24b are urged against the projections44a, 44b of levers 42a, 42b and thus, the pressing plate 40 is urgedagainst the stack of sheets 28 with a strong force caused by therestoring force of the coiled springs 46a, 46b and the gravitationalforce of the pressing plate 40, the arms 24a, 24b and the levers 42a,42b. As a result of this, the thick stack of sheets 28 can be positivelyfed one by one.

When the number of sheets of the stack 28 is small and the thickness ofthe sheet stack 28 becomes less than the predetermined value T_(R), theprojections 44a, 44b of levers 42a, 42b are in contact with the loweredges of openings 45a, 45b formed in the side walls 41a, 41b and thusthe levers 42a, 42b can not be further rotated in the counter-clockwisedirection in FIG. 5A. Therefore, the pressing plate 40 is urged againstthe stack of sheets 28 with a small force due to the gravitational forceof the plate 40 and the arms 24a, 24b, and the sheets can be effectivelyfed without causing undesired jamming.

FIG. 6 is a cross section depicting another embodiment of a sheetfeeding apparatus according to the invention. In this embodiment thesimilar portions as those shown in the previous embodiments aredesignated by the same reference numerals used in FIGS. 4A and 4B. Inthis embodiment, the rotational movement of an arm 24 in thecounter-clockwise direction about a shaft 23 is inhibited by a stopper35. A pressing roller 50 journaled by a shaft 33 comprises a resilientsleeve 51 made of foamed rubber. Therefore, after the arm 24 is incontact with the stopper 35, the stack of sheets 28 is pressed againstthe feeding roller 21 with a small force due to the restoring force ofthe deformed sleeve 51 of the pressing roller 50.

FIG. 7 is a cross section showing still another embodiment of the sheetfeeding apparatus according to the invention. In the present embodimentpressing means is formed by a sleeve like roller 55 having a hole 56 ofa relatively large diameter and a pin 57 secured to an arm 24 pivotedabout a shaft 23 is inserted in the hole 56 of the roller 55. When thethickness of a stack of sheets 28 is larger than the predetermined valueT_(R), the arm 24 is lifted upward by the engagement of the hole 56 andthe pin 57 and thus, the pressing roller 55 is urged against the stackedsheets 28 with a greater force due to the restoring force of the coiledspring 32 and the gravitational force of the roller 55. On the otherhand, when the thickness of the sheet stack 28 becomes lower than thepredetermined value T_(R), the rotation of the arm 24 is inhibited bythe stopper 35 and the roller 55 is urged against the stacked sheets 28with a smaller force due only to its gravitational force.

As explained above in detail, according to the invention the stack ofsheets can always be pressed against the feeding roller with a suitableforce which is varied automatically in accordance with the change inthickness of the stack of sheets, and therefore the sheets can be fedone by one from the lowermost one in a positive and safe manner withoutcausing undesired jamming of sheets regardless of the thickness of thestacked sheets.

It should be noted that the present invention is not limited to theembodiments explained above, but may be modified in various mannerswithin the scope of the invention. For instance, the coiled spring 32may be provided at a position between the arm 24 and the shaft 23, andsimilarly the coiled springs 46a, 46b may be provided at positionsbetween the levers 42a, 42b and the pins 43a, 43b, respectively.

What is claimed is:
 1. An automatic sheet feeding apparatuscomprising:means arranged below a stack of sheets for feeding the sheetsone by one from the lowermost sheet which is in contact with the feedingmeans; means arranged above the stack of sheets for pressing the stackof sheets against the feeding means with a first force when thethickness of the stack of sheets is larger than a predetermined value,and with a second force which is smaller than said first force when thethickness of the stack of sheets is at most equal to said predeterminedvalue, said pressing means including:a first pressing member which ismovable in a substantially vertical direction to produce said secondforce due to the gravitational force of the pressing member; a secondpressing member loosely coupled with said first pressing member forurging said first pressing member against the stack of sheets to producesaid first force; and a stopper member for inhibiting the operation ofsaid second pressing member after the thickness of said stack of sheetshas decreased to said predetermined value while permitting the operationof said first pressing member; means arranged at a downstream positionviewed in a sheet feed direction for separating the lowermost sheet fromthe stack of sheets to feed the thus separated sheet forward.
 2. Anautomatic sheet feeding apparatus comprising:means arranged below astack of sheets for feeding the sheets one by one from the lowermostsheet which is in contact with the feeding means; means arranged abovethe stack of sheets for pressing the stack of sheets against the feedingmeans with a first force when the thickness of the stack of sheets islarger than a predetermined value, and with a second force which issmaller than said first force when the thickness of the stack of sheetsis at most equal to said predetermined value, said pressing meansincluding:a first pressing member which is movable in a substantiallyvertical direction to produce said second force due to the gravitationalforce of the pressing member; a second pressing member loosely coupledwith said first pressing member for urging said first pressing memberagainst the stack of sheets to produce said first force; and a stoppermember for inhibiting the operation of said second pressing member afterthe thickness of said stack of sheets has decreased to saidpredetermined value; said first pressing member comprises a pressingroller, said second pressing member comprises a rotatable arm with whichsaid pressing roller is loosely coupled and a spring connected to thearm for rotating the arm in such a direction that the pressing roller isurged against the stack of sheets, and said stopper member is arrangedin a rotational locus of the arm at such a position that the arm is incontact with the stopper member when the thickness of the stack ofsheets has decreased to said predetermined value; means arranged at adownstream position viewed in a sheet feed direction for separating thelowermost sheet from the stack of sheets to feed the thus separatedsheet forward.
 3. An apparatus according to claim 2, wherein said armhas secured thereto a bearing having an elongated hole and said pressingroller has a shaft movably inserted in said elongated hole of thebearing.
 4. An apparatus according to claim 3, wherein said pressingroller is formed by a sleeve like roller having a central hole of alarge diameter and said arm has a pin of a small diameter inserted intosaid center hole of the sleeve like roller.
 5. An apparatus according toclaim 2, wherein said spring is formed by a coiled spring connectedbetween a free end of said rotatable arm and a fixed member of theapparatus.
 6. An apparatus according to claim 1, wherein said firstpressing member comprises a pressing plate and a rotatable arm to whichsaid pressing plate is secured, and said second pressing membercomprises a rotatable lever arranged beside said arm, a projectionconnected to a free end of the lever and extending into the rotationallocus of the arm, and a spring connected to the lever for rotating thelever in such a direction that the pressing plate is urged against thestack of sheets via an engagement with the projection and the arm, andsaid stopper member is provided in the rotational locus of the lever orprojection at such a position that the lever or projection is in contactwith the stopper member when the thickness of the stack of sheets hasdecreased to said predetermined value.
 7. An apparatus according toclaim 6, wherein said stopper member is formed by an opening formed in afixed member and said projection is extended through said opening.
 8. Anapparatus according to claim 6, wherein said spring is connected betweena free end of the lever and a fixed member.
 9. An apparatus according toclaim 1, wherein said pressing means comprises a pressing roller made ofresilient material, a rotatable arm supporting said pressing roller, aspring connected to a free end of said arm and a fixed member, and saidstopper member provided in the rotational locus of the arm at such aposition that after the thickness of the stack of sheets has decreasedto said predetermined value, the rotation of said arm is inhibited andthe stack of sheets is urged against the feeding means by said secondforce due only to the restoring force of said resilient pressing roller.10. An apparatus according to any one of claims 1 to 9, wherein saidfeeding means comprises a feeding roller which is rotated in a givendirection.
 11. An apparatus according to any one of claims 1 to 9,wherein said separating means comprises a separating roller rotated in agiven direction and being in contact with the lowermost sheet, apressing plate arranged above the separating roller in an inclinedfashion with respect to the sheet feed direction and a friction sheetapplied on that surface of said plate which faces the separating roller.